In pressurized process flow lines, it is often necessary to place a seal between a first surface and a second surface that slides along the first surface between two or more different static positions. In these circumstances, the seal often needs to meet multiple functional criteria. The seal needs to provide a strong static seal against one or both of the first and second surfaces, especially at the two or more different static positions, in order to prevent process fluid from seeping between the two surfaces. The seal also needs to maintain a dynamic seal against one or both of the surfaces to prevent process fluid from seeping between the two surfaces while the seal is sliding along the opposing seal surface. However, it is generally undesirable for the dynamic seal to impinge so strongly against the two surfaces so as to unduly impede movement of the first and second surfaces by excess friction.
FIG. 1 shows an example sliding stem valve 10 of a pressurized process flow line including a seal 12 that forms both a dynamic seal and a static seal. The sliding stem valve 10 includes a valve body 14 defining a flow passage 16 extending from an inlet 18 to an outlet 20, a bonnet 22 attached to the valve body 14, a flow control member 24 in the flow passage 16 and arranged to open and/or close the flow passage 16, and a valve stem 26 attached to the flow control member 24 and extending out through the bonnet 22 for operative coupling with a valve actuator (not shown). The flow control member 24 closes the flow passage 16 by sealingly engaging against a valve seat 28 surrounding the flow passage 16 in a closed position. The flow control member 24 opens the flow passage 16 by moving away from the valve seat 28 into an open position. The valve stem 26 slides up and down, i.e., reciprocates linearly, to move the flow control member 24 into and out of sealing engagement with the valve seat 28. A cage 30 in the form of a tubular, e.g., cylindrical, member surrounds the flow control member 24 to keep the flow control member 24 in alignment with the seat 28. The flow control member 24 is sized to fit inside the cage 30 such that the outer peripheral surface (e.g., an outside diameter) of the flow control member 24 is only slightly smaller than the inner peripheral surface (e.g., an inside diameter) of the cage 30, for example forming a gap therebetween, for example, of less than a few hundredths of an inch or of less than a few tenths of a millimeter.
The seal 12 is arranged to maintain a fluid seal in the gap between the flow control member 24 and the cage 30. In the past, it was customary for the seal 12 to be a ring seal, such as an o-ring, formed of resilient sealing material, such as rubber or a similar material, and with a circular or rectangular cross-section profile. The o-ring seal 12 is disposed in a land 32 that forms a groove in the outer peripheral surface of the flow control member 24. The seal 12 sealingly engages against both the outer peripheral surface of the flow control member 24 within the land 32 and the inner peripheral surface of the cage 30.
However, the o-ring seal 12 only provides a single sealing mechanism, i.e., the outermost and innermost diametrical surfaces of the o-ring, to provide both a static seal and a dynamic seal. Thus, an o-ring type seal 12 is limited in its ability to provide a strong static seal between the flow control member 24 and the cage 30 without also causing excessive frictional forces while forming a dynamic seal that could impair or impede the motion of the flow control member 24 between the open and closed positions inside the cage 32.